Probably a worn hydraulic pump, bad internal seals or possibly a plugged transmission filter. Any of these conditions can cause low hydraulic pressure to the clutches. When the engine speed slows, the pump also turns slower, allowing the pressure to drop a little, especially if there are worn parts. If it's not the filter, you'll probably need to rebuild the transmission. If it IS the filter, it usually means that there has been a lot of wear on the clutches, making a lot of debris that gets caught in the filter. Either way, it's not very good.